1. Technical Field
This invention relates broadly to mobile cellular communication systems and in particular, to advanced paging methods for paging subscribers in such a cellular communication system while minimizing the paging load on the system.
2. Related Art
The cellular communications industry has experienced very rapid growth in both the areas serviced and the number of subscribers. It is not uncommon in many metropolitan areas in North America for mobile switching centers to have service requests numbering in excess of 100,000 call attempts per hour. Each such mobile switching center typically services a network of 100 or more cells and keeps track of all mobile telephones currently operating in its service area (SA) via a database commonly referred to as a visitor location register. This register stores information regarding the services available to each subscriber, his home mobile switching center and the current status (e.g., active or inactive) of the mobile telephone.
The mobile switching center, however, does not usually have accurate information regarding the precise geographical location of each of the mobile telephones within the service area. Consequently, when a call is initiated to a mobile telephone believed to be within the service area, a page must be broadcast over the forward control channels (FOCCs), which are also known as the paging and access channels (PACH) to all cells in the service area. When the mobile telephone responds to the page message, the particular cell containing the mobile telephone is then identified from the reverse control channel used for the mobile response and the call is then set up. In the event there is no response to the page, the system assumes that the mobile telephone is currently inactive and the call is treated accordingly.
In view of the fact that each typical mobile switching center in an urban area serves a system with over 100 cells, in order to broadcast a page message over the forward control channels of all the cells in such a typical system, the paging message must be replicated a corresponding number of times and sent to each cell. As only one cell in the system can respond, this means that a large number of these paging messages are not required as they will result in no reply whatsoever. These unneeded paging messages are produced at the expense of the mobile switching center being available for performing another task. Hence, the overall system performance is degraded by the overhead caused by the generation of unneeded paging messages.
Various approaches have been suggested for overcoming this problem. In one such approach, the cellular system is partitioned into location areas which are smaller than the service area covered by the mobile switching center. Every time a mobile telephone leaves a location area and enters another location area it registers a location update that indicates the mobile telephone is a visitor in the area from which the mobile telephone is registered. As a result, should a call be initiated to that mobile telephone, the paging message needs to be sent only to the cells in the location area where the mobile telephone is "visiting" thereby dramatically reducing system loading associated with paging that particular mobile telephone. However, this approach does increase the number of registration activities as registration occurs each time an active mobile telephone moves from one location area to another which will occur more frequently than would be required in systems where registration occurs only when a mobile telephone is moved from one service area to another.
A variant of the above described paging approach is known as zone paging. In this approach, a two dimensional array is developed known as a Location Accuracy Matrix (LAM). Each entry (i,j) contains the number of page responses received from subscribers in cell j when the last known location of the subscriber was in cell i. The LAM data is then used to develop a probability matrix (p(i,j)) which is the probability that a subscriber is in cell j given the fact that its last known location was in cell i. The zone used for paging a subscriber whose last known location was in cell i is all the cells j for which the probability p(i,j)&gt;0.001. In the event that this selective zone page is unsuccessful, then all cells in the service area are paged in an attempt to locate the subscriber. If the all zone page is unsuccessful, the subscriber is assumed to have his or her mobile phone turned off.
Further attempts to reduce overall paging have proved somewhat successful. A more optimal approach has been developed where the inclusion of a cell j into a given zone Z(i) is based on whether inclusion of cell j increases or decreases the overall paging rather than on a single cut-off point of 0.001. This more optimal approach is accomplished by first determining the probability p(i,j) that a subscriber is in cell j given that its last known location was cell i and arrange these probabilities p(i,j) for all j in decreasing order and let the cells with decreasing probabilities be cells j.sub.1, j.sub.2, . . . j.sub.n. Next the zone Z(i) is initialized to contain no cells. For each k from 1 to n, include the cell j.sub.k in the zone if zone j.sub.k-1 is in the zone and the following inequality is true: EQU k+(1-(p(i,j.sub.1)+p(i,j.sub.2)+ . . . +p(i,j.sub.k)))*n&lt;=k-1+(1-(p(i,j.sub.1)+p(i,j.sub.2)+ . . . +p(i,j.sub.k-1)))*n
The process is stopped when a value of k is reached where the inequality fails. The more optimal zone Z(i) then consists of the cells j.sub.1, j.sub.2, j.sub.3, . . . j.sub.k-1. This more optimal approach does reduce paging compared to the earlier described zone paging approach, however, it does still leave considerable paging in the worst-case cell. The higher paging in the worst-case cell is due to the previously unrecognized consequence of having such worst-case cell in more than one zone.
However, such techniques of creating zones for zone paging often result in over-burdening of some of the cells. Such over-burdening causes same of similar drawbacks and problems as were previously caused. Thus, such prior techniques often failed as well to enhance operation in all cells of the network.